1. Field of the Invention
This invention relates to an optical multiplexing/demultiplexing device that performs multiplexing and demultiplexing of optical signals.
2. Description of the Related Art
In an optical subscriber loop system, it is necessary to perform, by a single optical fiber, optical transmission from a subscriber to a station, that is, uplink transmission, and optical transmission from the station to the subscriber, that is, downlink transmission. For that reason, between uplink transmission and downlink transmission, lights of different wavelengths are used. Consequently, an optical multiplexing/demultiplexing device that multiplexes/demultiplexes these lights of different wavelengths becomes necessary.
The optical multiplexing/demultiplexing device that is used on the subscriber side is called an optical network unit (ONU). Many ONUs that are presently used are configured from a wavelength filter, a photodiode and a laser diode whose optical axes are spatially optically combined. Further, an ONU that renders optical axis combination unnecessary by using optical waveguides is also known (e.g., see Japanese Patent Application Laid-Open (JP-A) No. 8-163028).
Further, in recent years, an ONU that uses, as a waveguide material, Si, which has excellent mass productivity, has garnered attention. As this type of ONU, an ONU that uses a Mach-Zehnder interferometer, an ONU that uses a directional coupler, and an ONU that uses a grating are known.
However, a Si-made ONU that uses a directional coupler is susceptible to wavelength shifts of the light source. Further, the device becomes a size of the order of several hundreds of μm, so it is difficult to make the device compact.
Further, in a Si-made ONU that uses a grating, it is necessary for the period of the grating to be equal to or less than half the wavelength, so it is difficult to fabricate the device.
Moreover, in a Si-made ONU that uses a Mach-Zehnder interferometer, wavelength dependence, such as the equivalent refractive index and the coupling coefficient of the directional coupler, is extremely large, so in the wavelength range that is used by the ONU, crosstalk arises and light intensity drops, so desired characteristics have been unable to be obtained.